1. Field of the Invention
This invention relates to the art of cleaning contamination such as old paint, grease, rust and the like from surfaces by blast cleaning. In particular, the invention is concerned with blast cleaning wherein relatively soft abrasive particles such as sodium bicarbonate particles are transported into impact engagement with the contaminated surface by a stream of pressurized air or water, and, more particularly, is concerned with novel means and methods of uniformly dispersing the soft abrasive particles into the pressurized air or water stream.
2. Summary of Prior Art
In recent years, there has been an increase in the use of cleaning systems utilizing a blast of abrasive sodium bicarbonate particles suspended in a stream of pressured air or water. Sodium bicarbonate as an abrasive blast media has distinct advantages over sand particles used for many years as the abrasive media for blast cleaning. Because of the toxic nature of sand particles (crystalline silica) when inhaled, government regulations require the use of sophisticated fresh air breathing masks to insure the health of the operator by preventing the ingestion of the silica product into the lungs. Sand blasting, moreover, cannot be economically utilized to clean softer substrates such as aluminum, plastic laminates and the like or used to blast clean machines in food processing plants because of the difficulty of removing the silica particles such as from bearing surfaces.
On the other hand, sodium bicarbonate or other like relatively soft abrasives having a Mohs hardness of less than 4.0 can effectively clean softer substrates such as aluminum or plastic components without harming the underlying surface. Importantly, sodium bicarbonate particles are reasonably soluble in water and can be readily removed by hosing down the machine and substrate after the blast cleaning. Sodium bicarbonate is not toxic and does not require elaborate fresh air breathing masks for the operator. Only standard protective clothing and ear and eye protection may be utilized. This is not necessarily a requirement but depends primarily on the substrate and the coating being removed. Sodium bicarbonate can be utilized to remove surface corrosion, lime, scale, paint, grease and machine oil from any surface, without damaging the surface and can be washed away from bearing surfaces of machinery.
Standard sand blasting equipment consists of a pressure vessel or blast pot to hold particles of sand, connected to a source of compressed air by means of a hose and having a means of metering the blasting medium from the blast pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 10-30 pounds per minute. About 1.2 pounds of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 1.20.
As discussed above, when it is required to remove coatings such as paint or to clean surfaces such as aluminum, magnesium, plastic composites and the like, less aggressive abrasives, including inorganic salts such as sodium chloride and sodium bicarbonate can be used in conventional sand blasting equipment. The media flow rates required for the less aggressive abrasives is substantially less than that used for sand blasting, and has been determined to be from about 0.5 to about 10.0 pounds per minute, using similar equipment. This requires much lower medium to air ratio, in the range of about 0.05 to 0.40.
The employment of less aggressive abrasives such as sodium bicarbonate as a blast cleaning medium does encounter problems in effecting the transfer of the abrasive particles from a supply hopper to the nozzle from which pressured water or air issues and where the abrasive is mixed into the pressured fluid. For example difficulties have been encountered in maintaining continuous flow of sodium bicarbonate particles at the low flow-rates used for this abrasive when conventional sand blasting equipment relying on gravity feed were employed. The fine particles of a medium such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, they tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. In an attempt to overcome these particle delivery problems, a sodium bicarbonate crystal has been developed and marketed under the trademark "ARMEX" by Church & Dwight Co., Inc. of Princeton, N.J. A flow additive such as hydrophobic silica has been applied to the sodium bicarbonate particles to promote the flow of the resulting crystals from the hopper and into the pressured stream of air or water passing through the discharge nozzle. Even this improved particle form of sodium bicarbonate still suffers from sporadic clogging and/or inconsistent rates of delivery of the sodium bicarbonate particles to the pressurized fluid stream, which in turn leads to erratic performance.
The methods and apparatus employed for delivering sodium bicarbonate or other less aggressive abrasive media have been improved by Church & Dwight and are the subjects of U.S. Pat. Nos. 5,081,799; 5,083,402 and 5,230,185 herein incorporated by reference. Briefly, as disclosed therein a high air pressure is maintained on the top of the mass of sodium bicarbonate particles disposed in the supply hopper to maintain a differential pressure between the top of the hopper and the air conveying line which directs the abrasive particulate to the blast nozzle which accelerates the particles to the substrate surface. Further, fine control of the flow of abrasive from the hopper to the conveying line is achieved by causing the abrasive to pass through an orifice. By controlling the differential pressure and size of the orifice, fine and exact control of abrasive flow has been obtained. Under these conditions, the sodium bicarbonate particles have been found to feed uniformly and consistently into a stream of pressured air or air and injected water. However, the feeding equipment is somewhat specialized, can be relatively expensive for certain common blast cleaning applications and has not specifically addressed adding the particles to a pressurized water stream used as the primary fluid carrier to the substrate.
There is therefore still a need for an improved method and apparatus for effecting blast cleaning through the utilization of less aggressive abrasives such as sodium bicarbonate particles, whether treated with a flow promotion agent or not, which will effect a more reliable and consistent delivery of such particles to the blast nozzle and which can be conveniently adjusted to accommodate a substantial range of particle sizes of abrasives.